1. Field of the Invention
The present invention relates to amplifier circuits, and more particularly to an apparatus and method which compensates for distortion in a power amplifier.
2. Background of the Related Art
Compared to an analog mobile communication system, a digital mobile communication system has a wider signal bandwidth which allows it to transmit more signals at a higher rate. In such a system, the signal is converted into a radio frequency (RF) signal and transmitted a certain distance. An RF power amplifier is used at a final transmission stage of the mobile communication system to enable the transmission.
In an RF power amplifier, linearity is a factor in amplifying every input signal at the same amplification rate for output. Unfortunately, most power amplifiers do not have a wide linearization range, so that a signal amplified at a region beyond the linearization range is inevitably distorted and outputted.
Methods for improving the distortion characteristics of power amplifiers have therefore been developed. These methods include a feed forward method, an envelope feedback method, and a pre-distortion method. Among these, the pre-distortion method is widely used because of its low cost for relatively high performance and operation within a wider bandwidth.
FIG. 1 shows an apparatus which compensates for pre-distortion in a power amplifier according to the related art. This apparatus includes a digital pre-distorter (DPD) 10 for pre-distorting a baseband digital signal (Vref) based on a specific compensation coefficient (Vcoef); a digital-to-analog converter (DAC) 20 for converting the digital signal (Vd) output from digital pre-distorter 10 into an analog signal (Vda); and an up-converter 30 for up-converting the baseband signal (Vda) outputted from the digital/analog converter 20 into a radio frequency signal (Vdf). The apparatus also includes a power amplifier (PA) 40 for amplifying the analog radio frequency signal (Vdf) outputted from the up-converter 30 and supplying the amplified signal to a directional coupler 50. Additionally, a down-converter 60 down-converts the analog radio frequency signal (Vfbrf) fed back after being branched from the directional coupler 50 into a baseband signal (VFBA). An analog-to-digital converter (ADC) 70 converts the analog signal (Vfba) outputted from the down-converter 60 into a digital signal (Vfb). And, a digital pre-distorter controller (DPDC) 80 compares the signal (Vfb) outputted from the analog-to-digital converter 70 and the baseband digital input signal (Vref) and generates a compensation coefficient (Vcoef) to compensate for the nonlinear characteristics of the power amplifier 40. The directional coupler 50 detects a portion (Vmrf) of the analog radio frequency signal outputted from the power amplifier 40 and transmits the remaining portion of the signal to the down-converter 60.
FIG. 2 shows digital pre-distorter 10 in accordance with the related art. The digital pre-distorter includes an instantaneous power measuring unit 11 for detecting a power level of the input digital signal (Vref), a look-up table 12 for storing a plurality of coefficient (Vcoef) applied from the digital pre-distorter controller 80 and outputting a compensation coefficient selected by a power level applied from the instantaneous power measuring unit 11, and a pre-distorter kernel 13 for pre-distorting the input digital signal (Vref) with the compensation coefficient transmitted from the look-up table 12. The resulting signal Vd is then sent to digital/analog converter 20.
FIG. 3 shows a method which compensates for pre-distortion of the power amplifier in accordance with the related art. Operation of the apparatus for compensating pre-distortion of a power amplifier shown in FIG. 1 may be described with reference to this method.
The apparatus for compensating pre-distortion of a power amplifier in accordance with the related art has roughly two operations modes: an initial mode for measuring nonlinear characteristics of power amplifier; and a normal mode for compensating an input signal with a compensation coefficient detected in the initial mode and outputting the compensated signal.
In the initial mode, a baseband digital signal is inputted as a training signal to the pre-distortion compensating apparatus (step S1). The training signal, which is used to detect nonlinear characteristics of the entire operation region of the power amplifier, is simultaneously applied to the digital pre-distorter 10 and the digital pre-distorter controller 80.
The digital pre-distorter 10 outputs an inputted digital signal as it is without pre-distorting it. The digital signal outputted from the digital pre-distorter 10 is converted into an analog baseband signal through the digital-to-analog converter 20, which is then applied to the up-converter 30 and up-converted into a radio frequency signal.
The analog radio frequency signal outputted from the up-converter 30 is amplified to a predetermined level in the power amplifier 40 and outputted therefrom (step S2). A portion of the output signal of the power amplifier 40 is detected by the directional coupler 50, and the remaining portion of the output signal is fed back to the down-converter 60. The signal inputted to the power amplifier 40 is distorted nonlinearly and outputted.
Upon receiving the analog radio frequency signal which has been branched at the directional coupler 50, the down-converter 60 down-converts it into a baseband signal and applies the baseband signal to the analog-to-digital converter 70. Then, the analog-to-digital converter 70 converts the received analog signal into a digital signal.
The digital pre-distorter controller 80 compares the digital signal outputted from the analog-to-digital converter 70 and the training signal which has been inputted to the digital pre-distorter 10 at the early stage (step S3), analyzes a distortion state of the signal according to the comparison result, generates a compensation coefficient, and applies the compensation coefficient (Vcoef) to the digital pre-distorter 10 (step S4).
The digital pre-distorter 10 classifies the compensation coefficient by power levels and stores it in the look-up table 12. In this case, the compensation coefficient is classified on the basis of an instantaneous power level of the digital signal inputted to the digital pre-distorter 10, and the power level serves as an address for outputting a compensation coefficient.
In this manner, the distortion state of the power amplifier 40 is checked in the initial mode. Then, when the compensation coefficient for linearizing the distortion state is completely stored in the look-up table of the digital pre-distorter 10, the apparatus for compensating pre-distortion of the power amplifier operates in the normal mode. In the normal mode, it is judged whether a signal inputted to the digital pre-distorter 10 is a normal signal (step S5).
If a normal signal is inputted to the digital pre-distorter 10 and to the digital pre-distorter controller 80, the instantaneous power measuring unit 11 of the digital pre-distorter 10 detects a power level of the input signal and applies it to the look-up table 12. Then, the look-up table 12 extracts a compensation coefficient by taking the power level as an address and applies the corresponding compensation coefficient to the pre-distorter kernel 13.
The pre-distorter kernel 13 pre-distorts the input signal and the compensation coefficient and outputs it to the digital-to-analog converter 20. An analog signal outputted from the digital-to-analog converter 20 is inputted to the power amplifier 40 after passing through the up-converter 30.
The analog signal inputted to the power amplifier 40 is a signal which has been compensated for the nonlinear characteristics of the power amplifier in the digital pre-distorter 10, so that the power amplifier 40 outputs a non-distorted, normal signal.
The output signal of the power amplifier 40 passes through the directional coupler 50, the down-converter 60 and the analog-to-digital converter 70 so as to be applied to the digital pre-distorter controller 80.
Then, the digital pre-distorter controller 80 analyzes again the fed back signal. If distortion occurs again newly, the digital pre-distorter controller 80 generates an updated compensation coefficient and records/stores it in the look-up table 12 of the digital pre-distorter 10 (step S7). The operation of obtaining a compensation coefficient and updating/storing it in the look-up table 12 while the pre-distortion compensating apparatus operates in the normal mode is called ‘adaptation’.
If a normal signal is successively inputted to the apparatus for compensating pre-distortion of the power amplifier 40 (step S8), the process of pre-compensating and detecting the input signal and updating the compensation coefficient is continually performed, and if there is no input signal, the process is terminated (step S9).
FIGS. 4A, 4B, 5A and 5B are graphs showing nonlinear characteristics of a general power amplifier. In detail, FIGS. 4A and 4B show gain characteristics and phase characteristics according to a temperature change and FIGS. 5A and 5B show gain characteristics and phase characteristics according to a frequency change.
In general, the nonlinear characteristics of the power amplifier can be roughly divided into AM-AM and AM-PM characteristics. The AM-AM characteristics is that gain characteristics are nonlinearly changed according to a size of a signal inputted to the power amplifier. The AM-PM characteristics is that a phase of an output signal is changed according to a size of a signal inputted to the power amplifier.
The AM-AM characteristics and the AM-PM characteristics cause generation of a spectral re-growth, so that when a signal is inputted to the power amplifier, the signal is distorted and outputted and the entire performance of a transmission system is deteriorated.
Therefore, in order to remove such nonlinear characteristics of the power amplifier, a pre-distorter is used. The pre-distorter improves the entire performance of a transmitter by constantly maintaining a gain of the power amplifier according to the size of an input signal and adjusting a phase of an output signal that is not changed according to the size of an input signal.
However, as shown in FIGS. 4A, 4B, 5A, and 5B, the nonlinear characteristics are also changed by a temperature or a frequency. Also, failure to compensate for the memory effect caused by those factors would also degrade the performance of the pre-distorter. Thus, the related art apparatus for compensating pre-distortion of the power amplifier has at least the following problems.
Since an input signal is simply pre-compensated by using a compensation coefficient, without considering a temperature change or a frequency change of an input signal, the pre-distorter cannot accurately compensate for the memory effect according to the temperature change or the frequency change of the input signal in the power amplifier.
Additionally, a signal inputted to the digital pre-distorter in the normal mode is not a training signal covering the entire operation region, so a compensation coefficient updated through the adaptation mode cannot properly compensate for the nonlinear characteristics of the power amplifier.